A complex containing p34cdc2 and cyclin B phosphorylates the nuclear lamin and disassembles nuclei of clam oocytes in vitro

ZP3 and AIPL1 participate in GVBD of mouse oocytes by affecting the nuclear membrane localization and maturation of farnesylated prelamin A

The low maturation rate of oocytes is an important reason for female infertility and failure of assisted pregnancy. The germinal vesicle breakdown (GVBD) is a landmark event of oocyte maturation. In our previous studies, we found that zona pellucida 3 (ZP3) was strongly concentrated in the nuclear region of germinal vesicle (GV) oocytes and interacted with aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) and lamin A to promote GVBD. In the current study, we found that lamin A is mainly concentrated in the nuclear membrane. When ZP3 is knocked down, lamin A will be partially transferred to the nucleus of oocytes. The prelamin A is increased in both the nuclear membrane and nucleus, while phosphorylated lamin A (p-lamin A) is significantly reduced. AIPL1 was also proved to accumulate in the GV region of oocytes, and ZP3 deletion can significantly inhibit the aggregation of AIPL1 in the nuclear region. Similar to ZP3 knockdown, the absence of AIPL1 resulted in a decrease in the occurrence of GVBD, an increase in the amount of prelamin A, and a significant decrease in p-lamin A in oocytes developed in vitro. Finally, we propose the hypothesis that ZP3 can stabilize farnesylated prelamin A on the nuclear membrane of AIPL1, and promote its further processing into mature lamin A, therefore promoting the occurrence of GVBD. This study may be an important supplement for the mechanism of oocyte meiotic resumption and provide new diagnostic targets and treatment clues for infertility patients with oocyte maturation disorder.

The LEM-ESCRT toolkit: Repair and maintenance of the nucleus

The eukaryotic genome is enclosed in a nuclear envelope that protects it from potentially damaging cellular activities and physically segregates transcription and translation.Transport across the NE is highly regulated and occurs primarily via the macromolecular nuclear pore complexes.Loss of nuclear compartmentalization due to defects in NPC function and NE integrity are tied to neurological and ageing disorders like Alzheimer’s, viral pathogenesis, immune disorders, and cancer progression.Recent work implicates inner-nuclear membrane proteins of the conserved LEM domain family and the ESCRT machinery in NE reformation during cell division and NE repair upon rupture in migrating cancer cells, and generating seals over defective NPCs. In this review, we discuss the recent in-roads made into defining the molecular mechanisms and biochemical networks engaged by LEM and many other integral inner nuclear membrane proteins to preserve the nuclear barrier.

PLK-1 promotes the merger of the parental genome into a single nucleus by triggering lamina disassembly

Life of sexually reproducing organisms starts with the fusion of the haploid egg and sperm gametes to form the genome of a new diploid organism. Using the newly fertilized Caenorhabditis elegans zygote, we show that the mitotic Polo-like kinase PLK-1 phosphorylates the lamin LMN-1 to promote timely lamina disassembly and subsequent merging of the parental genomes into a single nucleus after mitosis. Expression of non-phosphorylatable versions of LMN-1, which affect lamina depolymerization during mitosis, is sufficient to prevent the mixing of the parental chromosomes into a single nucleus in daughter cells. Finally, we recapitulate lamina depolymerization by PLK-1 in vitro demonstrating that LMN-1 is a direct PLK-1 target. Our findings indicate that the timely removal of lamin is essential for the merging of parental chromosomes at the beginning of life in C. elegans and possibly also in humans, where a defect in this process might be fatal for embryo development.

Expression and function of exportin 6 in full-grown and growing porcine oocytes

Exportin 6, which functions specifically in the nuclear export of actin family proteins, has been reported to be absent in immature Xenopus oocytes, which have a huge nucleus containing a large amount of actin. In mammalian oocytes, however, the presence and the function of exportin 6 remain uninvestigated. In this study, we assessed the expression and effects of exportin 6 on meiotic resumption in porcine oocytes after cloning porcine exportin 6 cDNA and carrying out overexpression and expression inhibition by mRNA and antisense RNA injection, respectively. We found for the first time that exportin 6 was expressed in mammalian full-grown germinal-vesicle-stage oocytes and was involved in the nuclear export of actin. In contrast, exportin 6 was absent from the growing oocytes, which are meiotically incompetent and maintain the germinal-vesicle structure in the long term; the regulatory mechanism appeared to be active degradation. We examined the effects of exportin 6 on meiotic resumption of porcine oocytes and noted that its expression did not affect the onset time but increased the rate of germinal vesicle breakdown at 24 h via regulation of the nuclear actin level, which directly influences the physical strength of the germinal-vesicle membrane. Our results suggest that exportin 6 affects the nuclear transport of actin and meiotic resumption in mammalian oocytes.

Dissecting the mechanisms of cell division

Cell division is a highly regulated and carefully orchestrated process. Understanding the mechanisms that promote proper cell division is an important step toward unraveling important questions in cell biology and human health. Early studies seeking to dissect the mechanisms of cell division used classical genetics approaches to identify genes involved in mitosis and deployed biochemical approaches to isolate and identify proteins critical for cell division. These studies underscored that post-translational modifications and cyclin-kinase complexes play roles at the heart of the cell division program. Modern approaches for examining the mechanisms of cell division, including the use of high-throughput methods to study the effects of RNAi, cDNA, and chemical libraries, have evolved to encompass a larger biological and chemical space. Here, we outline some of the classical studies that established a foundation for the field and provide an overview of recent approaches that have advanced the study of cell division.

Molecular Mechanisms of Prophase I Meiotic Arrest Maintenance and Meiotic Resumption in Mammalian Oocytes

Mechanisms of meiotic prophase I arrest maintenance (germinal vesicle [GV] stage) and meiotic resumption (germinal vesicle breakdown [GVBD] stage) in mammalian oocytes seem to be very complicated. These processes are regulated via multiple molecular cascades at transcriptional, translational, and posttranslational levels, and many of them are interrelated. There are many molecular cascades of meiosis maintaining and meiotic resumption in oocyte which are orchestrated by multiple molecules produced by pituitary gland and follicular cells. Furthermore, many of these molecular cascades are duplicated, thus ensuring the stability of the entire system. Understanding mechanisms of oocyte maturation is essential to assess the oocyte status, develop effective protocols of oocyte in vitro maturation, and design novel contraceptive drugs. Mechanisms of meiotic arrest maintenance at prophase I and meiotic resumption in mammalian oocytes are covered in the present article.

ZP3 is Required for Germinal Vesicle Breakdown in Mouse Oocyte Meiosis

ZP3 is a principal component of the zona pellucida (ZP) of mammalian oocytes and is essential for normal fertility, and knockout of ZP3 causes complete infertility. ZP3 promotes fertilization by recognizing sperm binding and activating the acrosome reaction; however, additional cellular roles for ZP3 in mammalian oocytes have not been yet reported. In the current study, we found that ZP3 was strongly expressed in the nucleus during prophase and gradually translocated to the ZP. Knockdown of ZP3 by a specific siRNA dramatically inhibited germinal vesicle breakdown (GVBD) (marking the beginning of meiosis), significantly reducing the percentage of MII oocytes. To investigate the ZP3-mediated mechanisms governing GVBD, we identified potential ZP3-interacting proteins by immunoprecipitation and mass spectrometry. We identified Protein tyrosine phosphatase, receptor type K (Ptprk), Aryl hydrocarbon receptor-interacting protein-like 1 (Aipl1), and Diaphanous related formin 2 (Diaph2) as potential candidates, and established a working model to explain how ZP3 affects GVBD. Finally, we provided preliminary evidence that ZP3 regulates Akt phosphorylation, lamin binding to the nuclear membrane via Aipl1, and organization of the actin cytoskeleton via Diaph2. These findings contribute to our understanding of a novel role played by ZP3 in GVBD.

Depletion of the LINC complex disrupts cytoskeleton dynamics and meiotic resumption in mouse oocytes

The SUN (Sad-1/UNC-84) and KASH (Klarsicht/ANC-1/Syne/homology) proteins constitute the linker of nucleoskeleton and cytoskeleton (LINC) complex on the nuclear envelope. To date, the SUN1/KASH5 complex is known to function as meiotic-specific factors. In this study, gene-silencing methods were used to explore the roles of SUN1 and KASH5 in mouse oocytes after prophase. SUN1 was detected throughout the nucleus; however, KASH5 was dispersed through the cell. After germinal vesicle breakdown (GVBD), SUN1 and KASH5 migrated during spindle formation and localized to the spindle poles at the MII stage. Most oocytes were arrested at the germinal vesicle (GV) stage after depletion of either SUN1 or KASH5. The DNA damage response was triggered in SUN1-depleted oocytes and thus gave rise to the G2/M checkpoint protein, p-CHK1. Oocytes that underwent GVBD had relatively small and abnormal spindles and lower levels of cytoplasm F-actin mesh. Immunofluorescence results also indicated the dislocation of pericentrin and P150^Glued after SUN1 or KASH5 depletion. Furthermore, KASH5 localized exclusively near the oocyte cortex after SUN1 depletion, but SUN1 localization was unaffected in KASH5-depleted oocytes. Taken together, the results suggest that SUN1 and KASH5 are essential factors in the regulation of meiotic resumption and spindle formation.

Caenorhabditis elegans polo-like kinase PLK-1 is required for merging parental genomes into a single nucleus

Before the first zygotic division, the nuclear envelopes of the maternal and paternal pronuclei disassemble, allowing both sets of chromosomes to be incorporated into a single nucleus in daughter cells after mitosis. We found that in Caenorhabditis elegans, partial inactivation of the polo-like kinase PLK-1 causes the formation of two nuclei, containing either the maternal or paternal chromosomes, in each daughter cell. These two nuclei gave rise to paired nuclei in all subsequent cell divisions. The paired-nuclei phenotype was caused by a defect in forming a gap in the nuclear envelopes at the interface between the two pronuclei during the first mitotic division. This was accompanied by defects in chromosome congression and alignment of the maternal and paternal metaphase plates relative to each other. Perturbing chromosome congression by other means also resulted in failure to disassemble the nuclear envelope between the two pronuclei. Our data further show that PLK-1 is needed for nuclear envelope breakdown during early embryogenesis. We propose that during the first zygotic division, PLK-1-dependent chromosome congression and metaphase plate alignment are necessary for the disassembly of the nuclear envelope between the two pronuclei, ultimately allowing intermingling of the maternal and paternal chromosomes.

Developmental Competence of Vitrified-Warmed Bovine Oocytes at the Germinal-Vesicle Stage is Improved by Cyclic Adenosine Monophosphate Modulators during In Vitro Maturation

Cryopreservation of mature oocytes and embryos has provided numerous benefits in reproductive medicine. Although successful cryopreservation of germinal-vesicle stage (GV) oocytes holds promise for further advances in reproductive biology and clinical embryology fields, reports regarding cryopreservation of immature oocytes are limited. Oocyte survival and maturation rates have improved since vitrification is being performed at the GV stage, but the subsequent developmental competence of GV oocytes is still low. The purpose of this study was to evaluate the effects of supplementation of the maturation medium with cyclic adenosine monophosphate (cAMP) modulators on the developmental competence of vitrified-warmed GV bovine oocytes. GV oocytes were vitrified-warmed and cultured to allow for oocyte maturation, and then parthenogenetically activated or fertilized in vitro. Our results indicate that addition of a cAMP modulator forskolin (FSK) or 3-isobutyl-1-methylxanthine (IBMX) to the maturation medium significantly improved the developmental competence of vitrified-warmed GV oocytes. We also demonstrated that vitrification of GV oocytes led to a decline in cAMP levels and maturation-promoting factor (MPF) activity in the oocytes during the initial and final phases of maturation, respectively. Nevertheless, the addition of FSK or IBMX to the maturation medium significantly elevated cAMP levels and MPF activity during IVM. Taken together, our results suggest that the cryopreservation-associated meiotic and developmental abnormalities observed in GV oocytes may be ameliorated by an artificial increase in cAMP levels during maturation culture after warming.

Phosphorylation of lamins determine their structural properties and signaling functions

Lamin A/C is part of the nuclear lamina, a meshwork of intermediate filaments underlying the inner nuclear membrane. The lamin network is anchoring a complex set of structural and linker proteins and is either directly or through partner proteins also associated or interacting with a number of signaling protein and transcription factors. During mitosis the nuclear lamina is dissociated by well established phosphorylation- dependent mechanisms. A-type lamins are, however, also phosphorylated during interphase. A recent study identified 20 interphase phosphorylation sites on lamin A/C and explored their functions related to lamin dynamics; movements, localization and solubility. Here we discuss these findings in the light of lamin functions in health and disease.

Vimentin as an integral regulator of cell adhesion and endothelial sprouting

Angiogenesis is a multistep process that requires intricate changes in cell shape to generate new blood vessels. IF are a large family of proteins that play an important structural and functional role in forming and regulating the cytoskeleton. Vimentin, a major type III intermediate filament protein is expressed in endothelial and other mesenchymal cells. The structure of vimentin is conserved in mammals and shows dynamic expression profiles in various cell types and different developmental stages. Although initial studies with vimentin-deficient mice demonstrated a virtually normal phenotype, subsequent studies have revealed several defects in cell attachment, migration, signaling, neurite extension, and vascularization. Regulation of vimentin is highly complex and is driven by posttranslational modifications such as phosphorylation and cleavage by intracellular proteases. This review discusses various novel functions which are now known to be mediated by vimentin, summarizing structure, regulation and roles of vimentin in cell adhesion, migration, angiogenesis, neurite extension, and cancer. We specifically highlight a pathway involving growth factor-mediated calpain activation, vimentin cleavage, and MT1-MMP membrane translocation that is required for endothelial cell invasion in 3D environments. This pathway may also regulate the analogous processes of neurite extension and tumor cell invasion.

Interphase phosphorylation of lamin A

Nuclear lamins form the major structural elements that comprise the nuclear lamina. Loss of nuclear structural integrity has been implicated as a key factor in the lamin A/C gene mutations that cause laminopathies, whereas the normal regulation of lamin A assembly and organization in interphase cells is still undefined. We assumed phosphorylation to be a major determinant, identifying 20 prime interphase phosphorylation sites, of which eight were high-turnover sites. We examined the roles of these latter sites by site-directed mutagenesis, followed by detailed microscopic analysis - including fluorescence recovery after photobleaching, fluorescence correlation spectroscopy and nuclear extraction techniques. The results reveal three phosphorylation regions, each with dominant sites, together controlling lamin A structure and dynamics. Interestingly, two of these interphase sites are hyper-phosphorylated in mitotic cells and one of these sites is within the sequence that is missing in progerin of the Hutchinson-Gilford progeria syndrome. We present a model where different phosphorylation combinations yield markedly different effects on the assembly, subunit turnover and the mobility of lamin A between, and within, the lamina, the nucleoplasm and the cytoplasm of interphase cells.

Studying lamins in invertebrate models

Lamins are nuclear intermediate filament proteins that are conserved in all multicellular animals. Proteins that resemble lamins are also found in unicellular organisms and in plants. Lamins form a proteinaceous meshwork that outlines the nucleoplasmic side of the inner nuclear membrane, while a small fraction of lamin molecules is also present in the nucleoplasm. They provide structural support for the nucleus and help regulate many other nuclear activities. Much of our knowledge on the function of nuclear lamins and their associated proteins comes from studies in invertebrate organisms and specifically in the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. The simpler lamin system and the powerful genetic tools offered by these model organisms greatly promote such studies. Here we provide an overview of recent advances in the biology of invertebrate nuclear lamins, with special emphasis on their assembly, cellular functions and as models for studying the molecular basis underlying the pathology of human heritable diseases caused by mutations in lamins A/C.

The Greatwall-PP2A axis in cell cycle control

Cell cycle progression is largely controlled by reversible protein phosphorylation mediated by cyclically activated kinases and phosphatases. It has long been known that cyclin B-Cdk1 activation triggers mitotic entry, and the enzymatic network controlling its activation and inactivation has been well characterized. Much more recently protein phosphatase 2A (PP2A) together with its B55 regulatory subunit has been recognized as the major activity dephosphorylating Cdk1 targets. Moreover, PP2A-B55 activity is high in late M phase and interphase, but low at mitotic entry. A series of discoveries in the fly and frog model systems have uncovered the molecular mechanism mediating this regulation. The Greatwall (Gwl) kinase activates endosulfines, which become specific inhibitors of PP2A-B55. Cdk1-dependent activation of Gwl at mitotic entry leads to PP2A-B55 downregulation, which synergizes with Cdk1 activation to promote the phosphorylated states of several mitotic substrates. Much less is known on the mechanisms inactivating Gwl and endosulfines at mitotic exit. Recent reports show the importance of spatiotemporal regulation of Gwl, endosulfines, and PP2A-B55 for cell cycle progression. The various systems and cell types differ in their dependence on the Gwl-PP2A axis for cell cycle progression. Moreover, this pathway also regulates gene expression in yeast, and this function could be conserved in metazoans.

Human immunodeficiency virus type 1 Vif causes dysfunction of Cdk1 and CyclinB1: implications for cell cycle arrest

The two major cytopathic factors in human immunodeficiency virus type 1 (HIV-1), the accessory proteins viral infectivity factor (Vif) and viral protein R (Vpr), inhibit cell-cycle progression at the G2 phase of the cell cycle. Although Vpr-induced blockade and the associated T-cell death have been well studied, the molecular mechanism of G2 arrest by Vif remains undefined. To elucidate how Vif induces arrest, we infected synchronized Jurkat T-cells and examined the effect of Vif on the activation of Cdk1 and CyclinB1, the chief cell-cycle factors for the G2 to M phase transition. We found that the characteristic dephosphorylation of an inhibitory phosphate on Cdk1 did not occur in infected cells expressing Vif. In addition, the nuclear translocation of Cdk1 and CyclinB1 was disregulated. Finally, Vif-induced cell cycle arrest was correlated with proviral expression of Vif. Taken together, our results suggest that Vif impairs mitotic entry by interfering with Cdk1-CyclinB1 activation.

FGF inhibits the activity of the cyclin B1/CDK1 kinase to induce a transient G₂arrest in RCS chondrocytes

Fibroblast growth factors (FGFs) negatively regulate long bone development by inhibiting the proliferation of chondrocytes that accumulate in the G₁ phase of the cycle following FGF treatment. Here we report that FGF also causes a striking but transient delay in mitotic entry in RCS chondrocytes by inactivating the cyclin B1-associated CDK1(CDC2) kinase. As a consequence of this inactivation, cells accumulate in the G₂ phase of the cycle for the first 4-6 hours of the treatment. Cyclin B1/CDK1 activity is then restored and cells reach a G₁ arrest. The reduced cyclin B1/CDK1 activity was accompanied by increased CDK1 inhibitory phosphorylation, likely caused by increased activity and expression of the Myt1 kinase. FGF1 also caused dephosphorylation of the CDC25C phosphatase, that however appears due the inactivation of cyclin B1/CDK1 complex in the CDK1 feedback loop, and not the activation of specific phosphatases. the inactivation of the cyclin B1/CDK1 complex is a direct effect of FGF signaling, and not a consequence of the G₂ arrest as it can be observed also in cells blocked at mitosis by Nocodazole. The Chk1 and AtM/ATR kinase are known to play essential roles in the G₂ checkpoint induced by DNA damage/genotoxic stress, but inhibition of Chk1 or ATM/ATR not only did not prevent, but rather potentiated the FGF-induced G₂ arrest. Additionally our results indicate that the transient G₂ arrest is induced by FGF in RCS cell through mechanisms that are independent of the G₁ arrest, and that the G₂ block is not strictly required for the sustained G₁ arrest but may provide a pausing mechanism that allows the FGF response to be fully established.

Nuclear lamins

The nuclear lamins are type V intermediate filament proteins that are critically important for the structural properties of the nucleus. In addition, they are involved in the regulation of numerous nuclear processes, including DNA replication, transcription and chromatin organization. The developmentally regulated expression of lamins suggests that they are involved in cellular differentiation. Their assembly dynamic properties throughout the cell cycle, particularly in mitosis, are influenced by posttranslational modifications. Lamins may regulate nuclear functions by direct interactions with chromatin and determining the spatial organization of chromosomes within the nuclear space. They may also regulate chromatin functions by interacting with factors that epigenetically modify the chromatin or directly regulate replication or transcription.

Characterization of the head-to-tail overlap complexes formed by human lamin A, B1 and B2 "half-minilamin" dimers

Half-minilamins, representing amino- and carboxy-terminal fragments of human lamins A, B1 and B2 with a truncated central rod domain, were investigated for their ability to form distinct head-to-tail-type dimer complexes. This mode of interaction represents an essential step in the longitudinal assembly reaction exhibited by full-length lamin dimers. As determined by analytical ultracentrifugation, the amino-terminal fragments were soluble under low ionic strength conditions sedimenting with distinct profiles and s-values (1.6-1.8 S) indicating the formation of coiled-coil dimers. The smaller carboxy-terminal fragments were, except for lamin B2, largely insoluble under these conditions. However, after equimolar amounts of homotypic amino- and carboxy-terminal lamin fragments had been mixed in 4 M urea, upon subsequent renaturation the carboxy-terminal fragments were completely rescued from precipitation and distinct soluble complexes with higher s-values (2.3-2.7 S) were obtained. From this behavior, we conclude that the amino- and carboxy-terminal coiled-coil dimers interact to form distinct oligomers (i.e. tetramers). Furthermore, a corresponding interaction occurred also between heterotypic pairs of A- and B-type lamin fragments. Hence, A-type lamin dimers may interact with B-type lamin dimers head-to-tail to yield linear polymers. These findings indicate that a lamin dimer principally has the freedom for a "combinatorial" head-to-tail association with all types of lamins, a property that might be of significant importance for the assembly of the nuclear lamina. Furthermore, we suggest that the head-to-tail interaction of the rod end domains represents a principal step in the assembly of cytoplasmic intermediate filament proteins too.

Nuclear lamins: key regulators of nuclear structure and activities

The nuclear lamina is a proteinaceous structure located underneath the inner nuclear membrane (INM), where it associates with the peripheral chromatin. It contains lamins and lamin-associated proteins, including many integral proteins of the INM, chromatin modifying proteins, transcriptional repressors and structural proteins. A fraction of lamins is also present in the nucleoplasm, where it forms stable complexes and is associated with specific nucleoplasmic proteins. The lamins and their associated proteins are required for most nuclear activities, mitosis and for linking the nucleoplasm to all major cytoskeletal networks in the cytoplasm. Mutations in nuclear lamins and their associated proteins cause about 20 different diseases that are collectively called laminopathies'. This review concentrates mainly on lamins, their structure and their roles in DNA replication, chromatin organization, adult stem cell differentiation, aging, tumorogenesis and the lamin mutations leading to laminopathic diseases.

Regulation of nuclear lamin polymerization by importin alpha

Nuclear lamins are integral components of the nuclear envelope and are important for the regulation of many aspects of nuclear function, including gene transcription and DNA replication. During interphase, the lamins form an intranuclear intermediate filament network that must be disassembled and reassembled when cells divide. Little is known about factors regulating this assembly/disassembly cycle. Using in vitro nuclear assembly and lamin assembly assays, we have identified a role for the nuclear transport factor importin alpha in the regulation of lamin assembly. Exogenous importin alpha inhibited nuclear lamin assembly in Xenopus interphase egg nuclear assembly assays. Fractionation of the egg extract used for nuclear assembly identified a high molecular weight complex containing the major egg lamin, XLB3, importin alpha, and importin beta. This complex could be dissociated by RanGTP or a competing nuclear localization sequence, indicating that lamin assembly is Ran- and importin alpha-dependent in the egg extract. We show that the addition of importin alpha to purified lamin B3 prevents the assembly of lamins in solution. Lamin assembly assays show that importin alpha prevents the self-association of lamins required to assemble lamin filaments into the typical paracrystals formed in vitro. These results suggest a role for importin alpha in regulating lamin assembly and possibly modulating the interactions of lamins with lamin-binding proteins.

Varicella-zoster virus infection of human foreskin fibroblast cells results in atypical cyclin expression and cyclin-dependent kinase activity

In its course of human infection, varicella-zoster virus (VZV) infects rarely dividing cells such as dermal fibroblasts, differentiated keratinocytes, mature T cells, and neurons, none of which are actively synthesizing DNA; however, VZV is able to productively infect them and use their machinery to replicate the viral genome. We hypothesized that VZV alters the intracellular environment to favor viral replication by dysregulating cell cycle proteins and kinases. Cyclin-dependent kinases (CDKs) and cyclins displayed a highly unusual profile in VZV-infected confluent fibroblasts: total amounts of CDK1, CDK2, cyclin B1, cyclin D3, and cyclin A protein increased, and kinase activities of CDK2, CDK4, and cyclin B1 were strongly and simultaneously induced. Cyclins B1 and D3 increased as early as 24 h after infection, concurrent with VZV protein synthesis. Confocal microscopy indicated that cyclin D3 overexpression was limited to areas of IE62 production, whereas cyclin B1 expression was irregular across the VZV plaque. Downstream substrates of CDKs, including pRb, p107, and GM130, did not show phosphorylation by immunoblotting, and p21 and p27 protein levels were increased following infection. Finally, although the complement of cyclin expression and high CDK activity indicated a progression through the S and G(2) phases of the cell cycle, DNA staining and flow cytometry indicated a possible G(1)/S blockade in infected cells. These data support earlier studies showing that pharmacological CDK inhibitors can inhibit VZV replication in cultured cells.

Invertebrate lamins

Lamins are the main component of the nuclear lamina and considered to be the ancestors of all intermediate filament proteins. They are localized mainly at the nuclear periphery where they form protein complexes with integral proteins of the nuclear inner membrane, transcriptional regulators, histones and chromatin modifiers. Studying lamins in invertebrate species has unique advantages including the smaller number of lamin genes in the invertebrate genomes and powerful genetic analyses in Caenorhabditis elegans and Drosophila melanogaster. These simpler nuclear lamina systems allow direct analyses of their structure and functions. Here we give an overview of recent advances in the field of invertebrate nuclear lamins with special emphasis on their evolution, assembly and functions.

Cyclin B2 suppresses mitotic failure and DNA re-replication in human somatic cells knocked down for both cyclins B1 and B2

Cyclin-dependent kinase 1 (CDK1) plays a crucial role in establishing metaphase and has also been shown to prevent DNA re-replication. Cyclins B1 and B2 are two known activators of CDK1 operating during mitosis in human cells. Little is known about the specific roles of each of these cyclins in CDK1 activation, but cyclin B2 is thought to play a minor role and to be unable to replace cyclin B1 for mitosis completion. In our study, we found that severe reduction by separate RNA interference of either cyclin B1 or cyclin B2 protein levels results in little or no alteration of the cell cycle and, more specifically, of mitosis progression. In contrast, simultaneous depletion of both B-type cyclins leads to massive accumulation of 4N cells, mitotic failure, premature mitosis exit and DNA re-replication. These defects can be corrected by the ectopic expression of a cyclin B2 resistant to the short hairpin RNA. Altogether, these data show that, in cycling human cells, cyclin B2 can compensate for the downregulation of cyclin B1 during mitosis. They also clearly implicate cyclins B1 and B2 as crucial activators of CDK1 in its biological function of DNA re-replication prevention.

Epstein-Barr virus BGLF4 kinase induces premature chromosome condensation through activation of condensin and topoisomerase II

Previous studies of Epstein-Barr virus (EBV) replication focused mainly on the viral and cellular factors involved in replication compartment assembly and controlling the cell cycle. However, little is known about how EBV reorganizes nuclear architecture and the chromatin territories. In EBV-positive nasopharyngeal carcinoma NA cells or Akata cells, we noticed that cellular chromatin becomes highly condensed upon EBV reactivation. In searching for the possible mechanisms involved, we found that transient expression of EBV BGLF4 kinase induces unscheduled chromosome condensation, nuclear lamina disassembly, and stress fiber rearrangements, independently of cellular DNA replication and Cdc2 activity. BGLF4 interacts with condensin complexes, the major components in mitotic chromosome assembly, and induces condensin phosphorylation at Cdc2 consensus motifs. BGLF4 also stimulates the decatenation activity of topoisomerase II, suggesting that it may induce chromosome condensation through condensin and topoisomerase II activation. The ability to induce chromosome condensation is conserved in another gammaherpesvirus kinase, murine herpesvirus 68 ORF36. Together, these findings suggest a novel mechanism by which gammaherpesvirus kinases may induce multiple premature mitotic events to provide more extrachromosomal space for viral DNA replication and successful egress of nucleocapsid from the nucleus.

Guilt by Association

The discovery that many inherited diseases are linked to interacting nuclear envelope proteins has raised the possibility that human genetic studies could be assisted by a fusion with proteomics. Two principles could be applied. In the first, the proteome of an organelle associated with a genetically variable disease is determined. The chromosomal locations of the genes encoding the organellar proteins are then determined. If a related disease is linked to a large chromosomal region that includes a gene identified in the organelle, then that gene has an increased likelihood of causing the disease. Directly sequencing this allele from patient samples might speed identification compared with further genetic linkage studies as has been demonstrated for multiple diseases associated with the nuclear envelope. The second principle is that if an organelle has been implicated in the pathology of a particular disorder, then comparison of the organelle proteome from control and patient cells might highlight differences that could indicate the causative protein. The distinct, tissue-specific pathologies associated with nuclear envelope diseases suggest that many tissues will have a set of disorders linked to this organelle, and there are numerous as yet unmapped or partially mapped syndromes that could benefit from such an approach.

Phosphorylation of the p34(cdc2) target site on goldfish germinal vesicle lamin B3 before oocyte maturation

The nuclear membranes surrounding fish and frog oocyte germinal vesicles (GVs) are supported by the lamina, an internal, mesh-like structure that consists of the protein lamin B3. The mechanisms by which lamin B3 is transported into GVs and is assembled to form the nuclear lamina are not well understood. In this study, we developed a heterogeneous microinjection system in which wild-type or mutated goldfish GV lamin B3 (GFLB3) was expressed in Escherichia coli, biotinylated, and microinjected into Xenopus oocytes. The localization of the biotinylated GFLB3 was visualized by fluorescence confocal microscopy. The results of these experiments indicated that the N-terminal domain plays important roles in both nuclear transport and assembly of lamin B3 to form the nuclear lamina. The N-terminal domain includes a major consensus phosphoacceptor site for the p34(cdc2) kinase at amino acid residue Ser-28. To investigate nuclear lamin phosphorylation, we generated a monoclonal antibody (C7B8D) against Ser-28-phosphorylated GFLB3. Two-dimensional (2-D) electrophoresis of GV protein revealed two major spots of lamin B3 with different isoelectric points (5.9 and 6.1). The C7B8D antibody recognized the pI-5.9 spot but not the pI-6.1 spot. The former spot disappeared when the native lamina was incubated with lambda phage protein phosphatase (lambda-PP), indicating that a portion of the lamin protein was already phosphorylated in the goldfish GV-stage oocytes. GFLB3 that had been microinjected into Xenopus oocytes was also phosphorylated in Xenopus GV lamina, as judged by Western blotting with C7B8D. Thus, lamin phosphorylation appears to occur prior to oocyte maturation in vivo in both these species. Taken together, our results suggest that the balance between phosphorylation by interphase lamin kinases and dephosphorylation by phosphatases regulates the conformational changes in the lamin B3 N-terminal head domain that in turn regulates the continual in vivo rearrangement and remodeling of the oocyte lamina.

Regulatory mechanisms and functions of intermediate filaments: a study using site- and phosphorylation state-specific antibodies

Intermediate filaments (IF) form the structural framework of the cytoskeleton. Although histopathological detection of IF proteins is utilized for examining cancer specimens as reliable markers, the molecular mechanisms by which IF are involved in the biology of cancer cells are still unclear. We found that site-specific phosphorylation of IF proteins induces the disassembly of filament structures. To further dissect the in vivo spatiotemporal dynamics of IF phosphorylation, we developed site- and phosphorylation state-specific antibodies. Using these antibodies, we detected kinase activities that specifically phosphorylate type III IF, including vimentin, glial fibrillary acidic protein and desmin, during mitosis. Cdk1 phosphorylates vimentin-Ser55 from prometaphase to metaphase, leading to the recruitment of Polo-like kinase 1 (Plk1) to vimentin. Upon binding to Phospho-Ser55 of vimentin, Plk1 is activated, and then phosphorylates vimentin-Ser82. During cytokinesis, Rho-kinase and Aurora-B specifically phosphorylate IF at the cleavage furrow. IF phosphorylation by Cdk1, Plk1, Rho-kinase and Aurora-B plays an important role in the local IF breakdown, and is essential for the efficient segregation of IF networks into daughter cells. As another part of our research on IF, we have set out to find the binding partners with simple epithelial keratin 8/18. We identified tumor necrosis factor receptor type 1-associated death domain protein (TRADD) as a keratin 18-binding protein. Together with data from other laboratories, it is proposed that simple epithelial keratins may play a role in modulating the response to some apoptotic signals. Elucidation of the precise molecular functions of IF is expected to improve our understanding of tumor development, invasion and metastasis.

Herpes simplex virus 1-encoded protein kinase UL13 phosphorylates viral Us3 protein kinase and regulates nuclear localization of viral envelopment factors UL34 and UL31

UL13 and Us3 are protein kinases encoded by herpes simplex virus 1. We report here that Us3 is a physiological substrate for UL13 in infected cells, based on the following observations. (i) The electrophoretic mobility, in denaturing gels, of Us3 isoforms from Vero cells infected with wild-type virus was slower than that of isoforms from cells infected with a UL13 deletion mutant virus (DeltaUL13). After treatment with phosphatase, the electrophoretic mobility of the Us3 isoforms from cells infected with wild-type virus changed, with one isoform migrating as fast as one of the Us3 isoforms from DeltaUL13-infected cells. (ii) A recombinant protein containing a domain of Us3 was phosphorylated by UL13 in vitro. (iii) The phenotype of DeltaUL13 resembles that of a recombinant virus lacking the Us3 gene (DeltaUs3) with respect to localization of the viral envelopment factors UL34 and UL31, whose localization has been shown to be regulated by Us3. UL34 and UL31 are localized in a smooth pattern throughout the nuclei of cells infected with wild-type virus, whereas their localization in DeltaUL13- and DeltaUs3-infected cells appeared as nuclear punctate patterns. These results indicate that UL13 phosphorylates Us3 in infected cells and regulates UL34 and UL31 localization, either by phosphorylating Us3 or by a Us3-independent mechanism.

Functions and dysfunctions of the nuclear lamin Ig-fold domain in nuclear assembly, growth, and Emery-Dreifuss muscular dystrophy

The non-alpha-helical C terminus of Xenopus lamin B3 (LB3T) inhibits the polymerization of lamin B3 in vitro and prevents the assembly of nuclei in Xenopus egg interphase extracts. To more precisely define the functions of LB3T in nuclear assembly, we have expressed subdomains of LB3T and determined their effects on nuclear assembly in Xenopus extracts. The results demonstrate that the Ig-fold motif (LB3T-Ig) is sufficient to inhibit lamin polymerization in vitro. Addition of the LB3T-Ig to egg extracts before the introduction of chromatin prevents chromatin decondensation and the assembly of the lamina, membranes, and pore complexes comprising the nuclear envelope. When added to assembled nuclei, LB3T-Ig prevents the further incorporation of lamin B3 into the endogenous lamina and blocks nuclear growth. The introduction of a point mutation in LB3T-Ig (R454W; LB3T-IgRW), known to cause Emery-Dreifuss muscular dystrophy when present in lamin A, does not inhibit lamin polymerization, chromatin decondensation, or nuclear assembly and growth. These results shed light on the specific alterations in lamin functions attributable to a known muscular dystrophy mutation and provide an experimental framework for revealing the effects of other mutations causing a wide range of laminopathies.

The nuclear lamina and its functions in the nucleus

The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of lamins, which are also present in the nuclear interior, and lamin-associated proteins. The increasing number of proteins that interact with lamins and the compound interactions between these proteins and chromatin-associated proteins make the nuclear lamina a highly complex but also a very exciting structure. The nuclear lamina is an essential component of metazoan cells. It is involved in most nuclear activities including DNA replication, RNA transcription, nuclear and chromatin organization, cell cycle regulation, cell development and differentiation, nuclear migration, and apoptosis. Specific mutations in nuclear lamina genes cause a wide range of heritable human diseases. These diseases include Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy (DCM) with conduction system disease, familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-Marie-Tooth disorder type 2, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg Skeletal Dysplasia, and Pelger-Huet anomaly (PHA). Genetic analyses in Caenorhabditis elegans, Drosophila, and mice show new insights into the functions of the nuclear lamina, and recent structural analyses have begun to unravel the molecular structure and assembly of lamins and their associated proteins.

Synchronization of interphase events depends neither on mitosis nor on cdk1

Human HT2-19 cells with a conditional cdk1 mutation stop dividing upon cdk1 inactivation and undergo multiple rounds of endoreplication. We show herein that major cell cycle events remain synchronized in these endoreplicating cells. DNA replication alternates with gap phases and cell cycle-specific cyclin E expression is maintained. Centrosomes duplicate in synchrony with chromosome replication, giving rise to polyploid cells with multiple centrosomes. Centrosome migration, a typical prophase event, also takes place in endoreplicating cells. The timing of these events is unaffected by cdk1 inactivation compared with normally dividing cells. Nuclear lamina breakdown, in contrast, previously shown to be dependent on cdk1, does not take place in endoreplicating HT2-19 cells. Moreover, breakdown of all other major components of the nuclear lamina, like the inner nuclear membrane proteins and nuclear pore complexes, seems also to depend on cdk1. Interestingly, the APC/C ubiquitin ligase is activated in these endoreplicating cells by fzr but not by fzy. The oscillations of interphase events are thus independent of cdk1 and of mitosis but may depend on APC/Cfzr activity.

SAPK/JNK regulates cdc2/cyclin B kinase through phosphorylation and inhibition of cdc25c

Cells undergo M phase arrest in response to stresses like UV irradiation or DNA damage. Stress-activated protein kinase (SAPK, also known as c-Jun N-terminal kinase, JNK) is activated by such stress stimuli. We addressed the potential effects of SAPK activation on cell cycle regulatory proteins. Activation of SAPK strongly correlated with inhibition of cdc2/cyclin B kinase, an important regulator of G2/M phase. SAPK directly phosphorylated the cdc2 regulator, cdc25c, in vitro on serine 168 (S168). This residue was highly phosphorylated in vivo in response to stress stimuli. cdc25c phosphorylated on S168 in cells lacks phosphatase activity, and expression of a S168A mutant of cdc25c reversed the inhibition of cdc2/cyclin B kinase activity by cell stress. Antibodies directed against phosphorylated S168 detect increased phosphorylation of S168 after cell stress. We conclude that SAPK regulates cdc2/cyclin B kinase following stress events by a novel mechanism involving inhibitory phosphorylation of the cdc2-activating phosphatase cdc25c on S168.

Specific inhibition of mouse oocyte nuclear protein phosphatase-1 stimulates germinal vesicle breakdown

Okadaic acid (OA)-induced germinal vesicle breakdown (GVBD) and localization of protein phosphatase-1 (PP1) in oocyte nuclei are suggestive of PP1's role in regulating oocyte GVBD. To explore this possibility, we microinjected protein phosphatase (PP) inhibitors OA, anti-PP1 antibody (anti-PP1), PP1 inhibitor I2, and anti-PP2A antibody (anti-PP2A) into nuclei of roscovitine (ROSC)-arrested mouse oocytes. Oocytes were also injected with recombinant PP1 in the absence of ROSC. Oocytes were assessed for GVBD and metaphase II (MII) development at 2 and 18 hr post-injection. Data were analyzed using Cochran-Mantel-Haenszel Statistics adjusted for time. Microinjection of OA significantly enhanced GVBD in comparison to controls at 2 and 18 hr (P < 0.01), yet had no effect on MII development. Similarly, microinjection of anti-PP1 resulted in significantly higher levels of GVBD compared to controls at 2 and 18 hr (P < 0.01). Interestingly, anti-PP1 microinjection also tended to enhance MII development at 18 hr in comparison to controls (P < 0.09). Microinjection of I2, anti-PP2A, and PP1 had no effect on GVBD or MII development. If reduction of PP1 activity was important for GVBD, one would anticipate an endogenous means of regulating PP1 activity at this developmental stage. In somatic cells, phosphorylation of PP1 at Thr320 causes PP1 inactivation. Germinal vesicle-intact oocytes did not contain phosphorylated PP1, as determined using a specific Thr320-Phospho-PP1 antibody, Western blot analysis, and confocal immunocytochemistry. At or around the time of GVBD, oocyte PP1 became phosphorylated at Thr320, which remained phosphorylated through MII development. These data indicate that inhibition of intra-nuclear PP1, through specific antibody neutralization, mimics OA-stimulated GVBD, providing the first direct evidence that nuclear PP1 is involved in regulation of oocyte nuclear membrane integrity. In addition, phosphorylation of PP1 occurs at/or around GVBD indicating that inactivation of PP1 is an important intracellular event in regulation of nuclear envelope dissolution at GVBD.

Mitotic reorganization of the intermediate filament protein nestin involves phosphorylation by cdc2 kinase

The intermediate filament protein nestin is expressed during early stages of development in the central nervous system and in muscle tissues. Nestin expression is associated with morphologically dynamic cells, such as dividing and migrating cells. However, little is known about regulation of nestin during these cellular processes. We have characterized the phosphorylation-based regulation of nestin during different stages of the cell cycle in a neuronal progenitor cell line, ST15A. Confocal microscopy of nestin organization and (32)P in vivo labeling studies show that the mitotic reorganization of nestin is accompanied by elevated phosphorylation of nestin. The phosphorylation-induced alterations in nestin organization during mitosis in ST15A cells are associated with partial disassembly of nestin filaments. Comparative in vitro and in vivo phosphorylation studies identified cdc2 as the primary mitotic kinase and Thr(316) as a cdc2-specific phosphorylation site on nestin. We generated a phosphospecific nestin antibody recognizing the phosphorylated form of this site. By using this antibody we observed that nestin shows constitutive phosphorylation at Thr(316), which is increased during mitosis. This study shows that nestin is reorganized during mitosis and that cdc2-mediated phosphorylation is an important regulator of nestin organization and dynamics during mitosis.

Identification and molecular cloning of germinal vesicle lamin B3 in goldfish (Carassius auratus) oocytes

A bulk isolation method was developed to collect a large number of germinal vesicles (GV) from postvitellogenic oocytes of goldfish (Carassius auratus). Using this method, we obtained GV lamina which are resistant to high salt and nonionic detergent. 2D PAGE revealed that the goldfish GV lamina contained several spots with similar molecular masses (67 kDa) and slightly different neutral isoelectrofocusing values (pI 5.8-6.2). After trypsin digestion and extraction of a major spot (pI 6.1), the peptide was subjected to RP-HPLC and sequenced. A homology search identified this spot as a nuclear lamin. A cDNA encoding goldfish GV lamin was isolated by RT-PCR using degenerate primers designed from the GV lamin tryptic peptide sequence. The goldfish GV lamin cDNA encodes a predicted molecular mass of 67 455 Da with a pI of 5.84. Phylogenetic analysis indicates that the amino-acid sequence is most similar to Xenopus oocyte-specific GV lamin B3, but differs from somatic lamins (A, B1 or B2). In contrast to somatic lamins, neither goldfish nor Xenopus GV lamin contain conserved phosphorylation sites for nuclear transport, except the nuclear localization sequence. Therefore, we conclude that the goldfish oocyte GV is mainly comprised of GV-type lamin (the homolog of Xenopus lamin B3).

Mechanisms of cell-cycle checkpoints: at the crossroads of carcinogenesis and drug discovery

Human tumors arise from multiple genetic changes that gradually transform growth-limited cells into highly invasive cells that are unresponsive to growth controls. The genetic evolution of normal cells into cancer cells is largely determined by the fidelity of DNA replication, repair, and division. Cell-cycle arrest in response to stress is integral to the maintenance of genomic integrity. The control mechanisms that restrain cell-cycle transition or induce apoptotic signaling pathways after cell stress are known as cell-cycle checkpoints. This review will focus on the mechanisms of cell-cycle checkpoint pathways and how different components of these pathways are frequently altered in the genesis of human tumors. As our knowledge of cell-cycle regulation and checkpoints increases, so will our understanding of how xenobiotic agents can affect these processes to either initiate or inhibit tumorigenesis.

Pneumocystis carinii uses a functional cdc13 B-type cyclin complex during its life cycle

Pneumocystis carinii causes severe pneumonia in immunocompromised patients. Recent studies indicate that P. carinii uses a Cdc2 cyclin-dependent kinase to control its proliferation. To further study the regulation of the life cycle of P. carinii, we characterized the P. carinii B-type cyclin termed Cdc13, whose binding to Cdc2 is necessary for kinase activity. Antibodies to B-type cyclins (Cdc13) specifically immunoprecipitated Cdc2/ Cdc13 complexes with associated kinase activity from P. carinii extracts. To clone P. carinii cdc13, degenerate polymerase chain reaction was undertaken using primers generated from amino-acid motifs conserved in fungal Cdc13 proteins. This amplicon was used to obtain full-length genomic and complementary DNA (cDNA) clones. A specific synthetic peptide antibody generated to P. carinii Cdc13 further demonstrated differential Cdc2/Cdc13 activity over the life cycle of P. carinii, with greater activity in cysts compared with trophic forms of the organism. Finally, P. carinii cdc13 cDNA was used to rescue mutant Schizosaccharomyces pombe strains containing temperature-sensitive deficiencies of endogenous Cdc13 activity, thus verifying function of the P. carinii Cdc13 protein. Therefore, P. carinii contains a Cdc13 cyclin, which is variably active over its life cycle and which promotes fungal proliferation.

Review: the dynamics of the nuclear lamins during the cell cycle-- relationship between structure and function

The nuclear lamins are members of the intermediate filament (IF) family of proteins. The lamins have an essential role in maintaining nuclear integrity, as do the other IF family members in the cytoplasm. Also like cytoplasmic IFs, the organization of lamins is dynamic. The lamins are found not only at the nuclear periphery but also in the interior of the nucleus, as distinct nucleoplasmic foci and possibly as a network throughout the nucleus. Nuclear processes such as DNA replication may be organized around these structures. In this review, we discuss changes in the structure and organization of the nuclear lamins during the cell cycle and during cell differentiation. These changes are correlated with changes in nuclear structure and function. For example, the interactions of lamins with chromatin and nuclear envelope components occur very early during nuclear assembly following mitosis. During S-phase, the lamins colocalize with markers of DNA replication, and proper lamin organization must be maintained for replication to proceed. When cells differentiate, the expression pattern of lamin isotypes changes. In addition, changes in lamin organization and expression patterns accompany the nuclear alterations observed in transformed cells. These lamin structures may modulate nuclear function in each of these processes.

Dynein is required for spindle assembly in cytoplasmic extracts of Spisula solidissima oocytes

Meiosis I spindle assembly is induced in lysate-extract mixtures prepared from clam (Spisula solidissima) oocytes. Unactivated lysate prepared from unactivated oocytes contain nuclei (germinal vesicles, GVs) which house condensed chromosomes. Treatment of unactivated lysate with clarified activated extract prepared from oocytes induced to complete meiosis by treatment with KCl induces GV breakdown (GVBD) and assembly of monopolar, bipolar, and multipolar aster-chromosome complexes. The process of in vitro meiosis I spindle assembly involves the assembly of microtubule asters and the association of these asters with the surfaces of the GVs, followed by GVBD and spindle assembly. Monoclonal antibody m74-1, known to react specifically with the N terminus of the intermediate chain of cytoplasmic dynein, recognizes Spisula oocyte dynein and inhibits in vitro meiosis I spindle assembly. Control antibody has no affect on spindle assembly. A similar inhibitory effect on spindle assembly was observed in the presence of orthovanadate, a known inhibitor of dynein ATPase activity. Neither m74-1 nor orthovanadate has any obvious affect on GVBD or aster formation. We propose that dynein function is required for the association of chromosomes with astral microtubules during in vitro meiosis I spindle assembly in these lysate-extract mixtures. However, we conclude that dynein function is not required for centrosome assembly and maturation or for centrosome-dependent aster formation.

Phosphorylation of the major Drosophila lamin in vivo: site identification during both M-phase (meiosis) and interphase by electrospray ionization tandem mass spectrometry

Phosphorylation can have profound effects on the properties of nuclear lamins. For instance, phosphorylation of specific sites on mammalian lamins drastically alters their propensity to polymerize. Relatively little is known about the effects of phosphorylation during interphase and about phosphorylation of invertebrate nuclear lamins. Here, using electrospray ionization tandem mass spectrometry, we determined the phosphorylation sites of both interphase and M-phase isoforms of nuclear lamin Dm from Drosophila melanogaster. Interphase lamins are phosphorylated at three sites: two of these sites (Ser25 and a site located between residues 430 and 438) flank the alpha-helical rod domain, whereas the third site (Ser595) is located close to the C-terminus. The M-phase lamin isoform is phosphorylated predominantly at Ser45, a residue contained within a sequence matching the consensus site for phosphorylation by cdc2 kinase. Our study confirms the important role in vivo for cdc2 kinase in M-phase disassembly of nuclear lamins and provides the basis for understanding Drosophila lamin phosphorylation during interphase.

Induction of germinal vesicle breakdown in a cell-free preparation from starfish oocytes

Incubation of isolated germinal vesicles in the homogenate from maturing starfish oocytes resulted in synchronous germinal vesicle breakdown (GVBD), and chromosome condensation and gathering within 30 min. GVBD in this cell-free system required aerobic conditions. The endogenous ATP-generation system was preserved in the homogenate and effective under aerobic conditions, and thus exogenous ATP was not added to the homogenate. Injection of the homogenate into immature starfish oocytes induced meiotic maturation without 1-methyladenine, indicating high activity of maturation-promoting factor (MPF) in the homogenate. MPF activity in the homogenate was stable for 2 h at room temperature, while it disappeared within 1 h in the supernatant prepared by centrifugation of the homogenate. This disappearance of MPF activity is regulated by cyclin B destruction, similar to that seen in vivo.

The plant cell cycle in context

Biological scientists are eagerly confronting the challenge of understanding the regulatory mechanisms that control the cell division cycle in eukaryotes. New information will have major implications for the treatment of growth-related diseases and cancer in animals. In plants, cell division has a key role in root and shoot growth as well as in the development of vegetative storage organs and reproductive tissues such as flowers and seeds. Many of the strategies for crop improvement, especially those aimed at increasing yield, involve the manipulation of cell division. This review describes, in some detail, the current status of our understanding of the regulation of cell division in eukaryotes and especially in plants. It also features an outline of some preliminary attempts to exploit transgenesis for manipulation of plant cell division.

Nuclear envelope disassembly in mitotic extract requires functional nuclear pores and a nuclear lamina

Using sea urchin embryonic and in-vitro-assembled nuclei incubated in sea urchin mitotic extract, I provide evidence for a requirement for functional nuclear pores and a nuclear lamina for nuclear envelope disassembly in vitro. In interphase gastrula nuclei, lamin B interacts with p56, an integral protein of inner nuclear membrane cross-reacting with antibodies to human lamin B receptor. Incubation of gastrula nuclei in mitotic cytosol containing an ATP-generating system rapidly induces hyperphosphorylation of p56 and lamin B. Subsequently, p56-lamin B interactions are weakened and the two proteins segregate into distinct nuclear envelope-derived vesicles upon disassembly of nuclear membranes and of the lamina. Nuclear disassembly is accompanied by chromatin condensation. Blocking nuclear pore function with wheat germ agglutinin or antibodies to nucleoporins prevents p56 and lamin B hyperphosphorylation, nuclear membrane breakdown and lamina solubilization. These events are not rescued by permeabilization of nuclear membranes to molecules of 150, 000 Mr with lysolecithin. In-vitro-assembled nuclei containing nuclear membranes with functional pores but no lamina do not disassemble in mitotic cytosol in spite of p56 hyperphosphorylation. Nuclear import of soluble lamin B and reformation of a lamina in interphase extract restores nuclear disassembly in mitotic cytosol. The data indicate a role for functional nuclear pores in nuclear disassembly in vitro. They show that p56 hyperphosphorylation is not sufficient for nuclear membrane disassembly in mitotic cytosol and argue that the nuclear lamina plays a critical role in nuclear disassembly at mitosis.

The regulation of mitotic nuclear envelope breakdown: a role for multiple lamin kinases

The chapter reviews the structure and function of the nuclear envelope and describes its dynamic structural changes during cell cycle. Particular emphasis is placed on the regulation of mitotic nuclear envelope breakdown (NEBD), the process by which the physical barrier between cytoplasm and nucleus is dissolved to allow for cell division. The literature suggesting the involvement of multiple protein kinases in NEBD is reviewed and evidence is presented that multiple mitotic lamin kinases, including p34cdc2/cyclin B kinase and protein kinase C, play key roles in mitotic nuclear lamina disassembly. Finally, a model for regulation of mitotic nuclear lamina disassembly by multi-site phosphorylation is described.

Role of phosphorylation in determining the backbone dynamics of the serine/threonine-proline motif and Pin1 substrate recognition

Proline residues provide a backbone switch in a polypeptide chain, which is controlled by the cis/trans isomerization about the peptidyl-prolyl bond. Phosphorylation of serine- and threonine-proline motifs has been shown to be a critical regulatory event for many proteins. The biological significance of these motifs has been further highlighted by the discovery of a novel and essential peptidyl-prolyl cis/trans isomerase Pin1. Pin1 is required for progression through mitosis via catalyzing the isomerization of phosphorylated Ser/Thr-Pro motifs specifically present in mitosis-specific phosphoproteins. However, little is known whether the phosphorylation regulates the conformational switch of the Ser/Thr-Pro bonds. Here, we report the synthesis and conformational characterization of a series of peptides that contain the phosphorylated or nonphosphorylated Ser/Thr-Pro motifs. Phosphorylation affected the rate of the cis to trans isomerization of the Thr/Ser-Pro bonds. As determined by a protease-coupled assay, the isomerization rate of phosphorylated Thr-Pro bond was found to be 8-fold slower than that of the nonphosphorylated analogue. Furthermore, studies of the pH dependence of the isomerization of the phosphopeptides reveal that both cis content and the rate constant of prolyl cis to trans isomerization are lower for the dianionic state of the phosphothreonine-containing peptides. These effects of phosphorylation are specific for phosphorylated Ser/Thr since neither phosphorylated Tyr nor glutamic acid was able to affect the prolyl isomerization. Finally, our experiments provide evidence that effective catalysis of cis/trans isomerization of phosphorylated Ser/Thr-Pro bonds by Pin1 is specific to the dianionic form of the substrate. Thus, our results demonstrate that protein phosphorylation specifically regulates the backbone dynamics of the Ser/Thr-Pro motifs and that Pin1 specifically isomerizes the certain conformation of the phosphorylated Ser/Thr-Pro motifs.

Reversal of myogenic terminal differentiation by SV40 large T antigen results in mitosis and apoptosis

Terminally differentiated skeletal muscle myotubes are arrested in the G0 phase of the cell cycle, and this arrest is not reversed by stimulation with serum or growth factors. The myotubes have been shown to be refractory to apoptosis even under low serum conditions. When the SV40 large T antigen is induced in the C2SVTts11 myotubes, which stably harbor the T antigen gene linked to an inducible promoter, the terminally differentiated cells reenter the cell cycle to resume nuclear DNA replication representing S phase. We show here that the large T-expressing myotubes further proceeded to M phase represented by the appearance of mitotic figures with centrosomes, condensed chromosomes, and mitotic spindles. The myotubes eventually cleaved and midbodies were formed at the cleavage sites of the cytoplasm. In some cases actin filaments, reminiscent of the contractile rings, accumulated at the cleavage furrows. Thus, terminally differentiated myotubes remain able to resume at least one round of the cell cycle and consequently are considered to be capable of dedifferentiation. A subset of myotubes expressing large T did not undergo mitosis. Some of them were degenerative and contained deformed giant nuclei and pulverized nuclei. The others suffered apoptotic cell death, which was identified by morphological changes of the nuclei and the labeling with dUTP at the ends of chromatin DNA fragments. The induction of apoptosis was unlikely to be confined to a particular phase of the cell cycle. These results imply that terminally differentiated myotubes also retain a complete set of machinery for apoptosis.

Nuclear envelope dynamics during male pronuclear development

Upon fertilization, the sperm nucleus undergoes reactivation. The poreless sperm nuclear envelope is replaced by a functional male pronuclear envelope and the highly compact male chromatin decondenses. Here some recent evidence is examined: that disassembly of the sperm lamina is required for chromatin decondensation, that remnant portions of the sperm nuclear envelope target the binding of egg membrane vesicles that form the male pronuclear envelope, that functional male pronuclear envelopes containing lamin B receptor assemble prior to lamin import and lamina formation, and that lamina assembly drives male pronuclear swelling. Several unresolved issues are discussed.